1. Field of the Invention
The present invention relates to a television receiver capable of receiving a television signal or a frequency-modulation (FM) broadcast signal.
2. Description of the Related Art
FIG. 8 shows the circuitry of a conventional television receiver. A tuning circuit and an amplifier that are not shown are deposited in stages preceding a mixer 31. A television signal or a FM broadcast signal selected by the tuning circuit is applied to the mixer 31. The mixer 31 uses a local-oscillator signal sent from a local oscillator 32 to convert the frequency of the television signal into an intermediate frequency band (ranging from 54 MHz to 60 MHz under the Japanese specifications for channels). Moreover, the frequency of the FM broadcast signal is converted into a sound intermediate frequency or a nearby frequency on the intermediate frequency band. The frequency-converted television signal or FM broadcast signal is applied to a first intermediate-frequency tuning circuit 33.
The first intermediate-frequency tuning circuit 33 is realized with a parallel tuning circuit. During reception of a television signal, the first intermediate-frequency tuning circuit 33 is tuned to a frequency between a video intermediate frequency and a color subcarrier frequency on the intermediate frequency band. During reception of a FM broadcast signal, the first intermediate-frequency tuning circuit 33 is tuned to the sound intermediate frequency or a nearby frequency. Switching the frequencies to which the first intermediate-frequency tuning circuit is tuned is controlled using a switching transistor 34. The switching transistor 34 has the emitter thereof grounded. A supply voltage B is applied to the collector of the switching transistor 34 via a pull-up resistor 35. The collector of the switching transistor 34 is connected to a control terminal 33a of the first intermediate-frequency tuning circuit 33. A high-level or low-level switching voltage S is applied to the base of the switching transistor 34.
The television signal or FM broadcast signal having passed through the first intermediate-frequency tuning circuit 33 is amplified by an intermediate-frequency amplifier 36 deposited in a subsequent stage. Thereafter, the resultant signal is applied to a second intermediate-frequency tuning circuit 37. The second intermediate-frequency tuning circuit 37 consists mainly of a series circuit composed of a first capacitive element 37a, a first inductive element 37b, and a second capacitive element 37c one of whose terminals is grounded, and a switching diode 37d connected in parallel with the first capacitive element 7a. The anode of the switching diode 37d is connected to the output terminal of the intermediate-frequency amplifier 36. A bias voltage produced by the intermediate-frequency amplifier 36 is applied to the anode of the switching diode 37d. Moreover, a node between the first inductive element 37b and a second capacitive element 37c is connected to the collector of the switching transistor 34.
In the second intermediate-frequency tuning circuit 37, a resonant frequency at which a circuit composed of the first capacitive element 37a, first inductive element 37b, and second capacitive element 37c becomes resonant with the switching diode 37d brought to non-conduction is set to a frequency between the video intermediate frequency and color subcarrier frequency on the intermediate frequency band. A resonant frequency at which a circuit composed of the first inductive element 37b and second capacitive element 37c becomes resonant with the switching diode 37d brought to conduction is set to the sound intermediate frequency on the intermediate frequency band or a nearby.
A first surface acoustic wave (SAW) filter 38 is connected to the node between the first inductive element 37b and second capacitive element 37c. Moreover, a second SAW filter 40 is connected to the node via a second inductive element 39. The second inductive element 39 is used to make the coupling between the first SAW filter 38 and second SAW filter 40 coarse, and offers a relatively large inductance (4.7 μH).
The first SAW filter 38 exhibits a bandpass response characteristic. Namely, the first SAW filter 38 substantially uniformly passes signal components whose frequencies lie between the video intermediate frequency and color subcarrier frequency on the intermediate frequency band, and attenuates a signal component of the sound intermediate frequency as largely as at about 20 dB. Moreover, the second SAW filter 40 has the characteristic of passing signal components whose frequencies fall within a narrow band so that the second SAW filter 40 will pass a signal component of the sound intermediate frequency or a nearby frequency so as to avoid interference with a FM broadcast signal on an adjoining channel.
A video intermediate-frequency circuit 41 is connected to the output terminal of the first SAW filter 38, and a video detection circuit (not shown) is disposed in a subsequent stage. Moreover, a sound intermediate-frequency circuit 42 is connected to the output terminal of the second SAW filter 40, and a sound detection circuit (not shown) is disposed in a subsequent stage.
In the foregoing circuitry, during reception of a television signal, the switching transistor 34 and switching diode 37d are brought to conduction. The first intermediate-frequency tuning circuit 33 and second intermediate-frequency tuning circuit 37 are tuned to a frequency between the video intermediate frequency and color subcarrier frequency on the intermediate frequency band. Consequently, the frequency-converted television signal is transferred via the first SAW filter 38 and video intermediate-frequency circuit 41.
During reception of a FM broadcast signal, the switching transistor 34 and switching diode 37d are brought to conduction. The first intermediate-frequency tuning circuit 33 and second intermediate-frequency tuning circuit 37 are tuned to the sound intermediate frequency on the intermediate frequency band or a nearby frequency. Consequently, the frequency-converted FM broadcast signal is transferred via the second SAW filter 40 and sound intermediate-frequency circuit 42.
In the aforesaid conventional television receiver, the frequency of a received FM broadcast signal is converted into the sound intermediate frequency on the intermediate frequency band or a nearby frequency. In order to avoid interference with a FM broadcast signal on an adjoining channel, the second SAW filter 40 is included to pass only the FM broadcast signal that should be received. Therefore, the television receiver has the drawback of high cost.
In order to prevent the level of a television signal applied to the first SAW filter 38 from dropping because of the presence of the second SAW filter 40, the second inductive element 39 offering a relatively large inductance is included for making the coupling between the first SAW filter 38 and second SAW filter 40 coarse. This is disadvantageous in terms of a compact design.